In recent years, composite materials have been produced using a carbon fiber, an aromatic polyamide fiber or the like as a reinforcing fiber. These composite materials, as compared with materials using no reinforcing material, are high in strength and rigidity. Utilizing the high strength and high rigidity, composite materials have been used in a large amount as a structural material for aircraft, etc.
Epoxy resin-based prepregs use an epoxy resin as a matrix resin to be impregnated into a reinforcing fiber. By using an epoxy resin whose main components are an aromatic glycidylamine-based epoxy resin (a main component) and diaminodiphenylsulfone (a curing agent), there can be produced a composite material excellent in heat resistance, mechanical properties, dimensional stability, chemical resistance and weather resistance.
Composite materials produced using an epoxy resin-based prepreg have been known to show good properties, but show short-term storage stability. Further, in these composite materials produced using such a prepreg, the matrix resin has small elongation and is brittle. Since the matrix resin has small elongation and is brittle, the composite material obtained is inferior in toughness and impact resistance. Therefore, it is required to improve conventional composite materials in impact resistance without impairing their heat resistance.
Particularly when such a composite material is used as a primary structural material for aircraft, the composite material undergoes, in some cases, mechanical impact when the aircraft hits small stones in take-off or landing or a tool is accidentally dropped on the aircraft during the maintenance. Therefore, improvement in impact resistance without reduction in heat resistance is an important task for epoxy resin-based composite material.
When a prepreg is cured to produce a composite material (a shaped material) of high impact resistance, it is naturally important to improve the reinforcing fiber (e.g. carbon fiber) itself in elongation. Meanwhile, it is also important to increase the toughness of the matrix resin constituting the composite material. Therefore, a number of attempts have been made for improvement of the matrix resin.
For improvement of the toughness of the matrix resin of composite material, there are considered, for example, a method of mixing a rubber component into the raw material epoxy resin and a method of mixing a high-molecular resin component into the epoxy resin. In the method of mixing a rubber component into the epoxy resin, the composite material obtained is improved in toughness and impact resistance. However, there is reduction in mechanical properties such as heat resistance, compression property, interlaminar shear property. As a result, the amount of the rubber component mixed is restricted and is small depending upon the application of the composite material obtained and, in this case, no sufficient improvement in toughness and impact resistance is obtained.
For mixing, into the raw material epoxy resin, a thermoplastic resin as a high-molecular resin component, there are a mixing method of dissolving a thermoplastic resin in a high-temperature epoxy resin and a mixing method of dissolving a thermoplastic resin in a solvent and then adding an epoxy resin thereto.
In the mixing method of dissolving a thermoplastic resin in a high-temperature epoxy resin, a sudden viscosity increase takes place when the concentration of the thermoplastic resin gets high and also a reduction in tackiness takes place. As a result, the operation of prepreg production becomes very poor.
In the mixing method of using a solvent, the removal of solvent after mixing is a problem. There are further problems, for example, the preparation of solution is complicated and the composite material obtained is low in heat resistance owing to the solvent remaining in a small amount.
For the above reasons, there has been employed a method of adding, to a matrix resin of prepreg, a small amount of a rubber component and a small amount of a high-molecular resin component to improve the prepreg in impact resistance. This method is low reduction in heat resistance but is slight improvement in impact resistance.
In Patent Literatures 1 to 3 are disclosed resin compositions in which a thermoplastic resin is dispersed in an epoxy resin in order to obtain a composite material of high toughness (high impact resistance), and prepregs produced using such resin compositions. However, the composite materials obtained are not improved in impact resistance to a satisfactory level.
The above resin compositions and prepregs contain diaminodiphenylsulfone as a curing agent for epoxy resin, in order to allow the composite material obtained to have high heat resistance. In this case of using the above curing agent, each resin composition and each prepreg obtained have short-term storage stability of about 2 to 3 weeks at room temperature (23° C.). Hence, in order to improve their handleability, high storage stability is required.
In Patent Literature 4 is disclosed a technique of using microencapsulated diaminodiphenylsulfone to make longer the storage stability of prepreg. However, use of a large amount of a thermoplastic resin gives a resin composition of high viscosity, making difficult the production of prepreg. Accordingly, the amount of thermoplastic resin in resin composition becomes 40 mass % or less, and use of a higher amount is difficult practically. Owing to the above reasons, there is a limit in improvement of the impact resistance of composite material.    Patent Literature 1: JP-A-1986-250021 (Claims)    Patent Literature 2: JP-A-1987-57417 (Claims)    Patent Literature 3: JP-A-1988-162732 (page 3, left lower column, line 9 from above to line 1 from below, page 3, right lower column, line 7 from above to page 4, left upper column, line 4 from above)    Patent Literature 4: JP-A-1992-249544 (Claims)